In recent years, in association with improvement in the field of image forming technology, in some cases a copied bill prepared by a digital color copying machine can not easily be discriminated from an actual bill. For this reason, a digital color copying machine, which can discriminate specific documents such as marketable securities or bills inhibited to be copied and prevent the specific documents from being copied illegally, has been put into actual use.
As a method of discriminating specific documents such as bills or marketable securities, there has been proposed, for instance, a method in which an inputted image data is compared to a special marking (pattern data) previously registered by means of the pattern matching method and a document is recognized as a specific document when the special marking is present in the inputted image data, or a method disclosed in Japanese Patent Laid Open Publication No. 54681/1992 in which a specific document is discriminated by comparing a form of a histogram based on color phase distribution to a standard one.
In company offices or the like, there exist various types of general documents, other than the specific document such as bills or marketable securities as described above, inhibited to be copied because of the importance of contents thereof or for security (described a copying-inhibited document hereinafter). Generally, this type of copying-inhibited document has a stamp indicating security or a marking indicating inhibition of copying thereon, which makes it possible for the document to be discriminated from other documents which may be copied.
In the methods as described above, however, a stamp indicating security or a marking indicating inhibition of copying is put on a copying-inhibited document to alert users that the document is inhibited to be copied, and in these cases inhibition of copying the document is preserved only when the users follow the rule, and copying is not physically inhibited, so that the users can copy the document by using, for instance, a copying machine, send the document by means of facsimile transmission using a facsimile device, or input an image of the document into an image filing device for storage.
It is conceivable that a device for inhibiting a copying-inhibited document to be copied is built by applying the conventional type of methods for discriminating specific documents to discrimination of copying-inhibited documents, but in a case where copying of a copying-inhibited document is inhibited by discriminating a stamp indicating security or a marking indicating inhibition of copying, for instance, by means of pattern matching method, if the stamp indicating security or marking indicating inhibition of copying is hid with paper or the like, discrimination can not be carried out, and it is impossible to make users preserve inhibition of copying a copying-inhibited document.
Also in a case where discrimination carried out by comparing a form of histogram based on color phase distribution to a standard one, there are problems that the method can not be applied to a device which treats image data as binary data for white and black, and that, as there are various types of copying-inhibited document and there is not always a common feature among the forms of histogram each based on color phase distribution, discrimination of a copying-inhibited document itself can not be carried out accurately.
Also the methods as described above assume that a mechanism (such as a board) for discriminating image data is always working when image read is executed, and if an error is generated in this mechanism, all the operations for copying must be inhibited, and a copying operation can not be executed until the error is removed, which disadvantageously gives damages or causes troubles in users' job or the like. This is especially fatal to users who must make many sheets of various documents or copy documents frequently.